Treating reservoir matrix



May 21, 1968 1. J. DAY ETAL TREATING RESERVOIR MATRIX Filed July 21,1966 INVEN fo/:ww J. DAYTORS United States Patent C) 3,384,177 TREATINGRESERVOIR MATRIX l Johnny J. Day, Pittsburgh, `limmie L. Huitt,Glenshaw, and Bruce B. McGlothlin, Pittsburgh, Pa., assignors to GulfResearch & Development Company, Pittsburgh, Pa., a corporation ofDelaware Fiied July 21, 1966, Ser. No. 566,957 12 Claims. (Cl. 166--42)This invention relates to a method of conditioning reservoir formationsto reduce the embedment of fracture proppants.

One of the primary methods of stimulating production in the oil industryis to hydraulically fracture the formation. Gwing to the greatoverburden pressures, these fractures must be mechanically held in anopen psition to prevent the 4closure of the fractures. Proppants areinjected into the fractures to maintain an open fracture.

At a time subsequent to the initiation of a successful hydraulicfracture treatment, the flow capacity may seriously decrease to theextent that `a further remedial Workover of the well is advisable. Oneof the primary methods utilized in the oil industry to increase the owcapacity of a well which has previously been fractured is tohydrothermally treat the formation, Owing to the various expandableminerals contained in hydrocarboncontaining formations, suchhydrothermal treatment will often soften the reservoir matrix to theextent that proppants will embed in the fracture face thereby permittingclosure of the fracture.

Methods heretofore employed to prevent fracture closure duringsubsequent hydrothermal treatments have been to anticipate such closureand inject a sufficient -amount of proppant to pack the fracture. Such amultilayered packed fracture will reduce the flow capacity of ythefracture in a like manner as encountered when proppants embed into thefracture faces. In addition, the utilization of a multilayer ofproppants will create a well condition favoring possible screen out ofthe proppant during injection. Dangerous excessive pressures may beapplied to the well bore when a sand screens out during fracturetreatment. The attempt to form a multilayer of proppants in the fractureis also associated with increased sand clean out after the treatment iscompleted.

Owing to the reduction of flow capacity when fractures `are packed witha multilayer of proppants, the oil industry has found that fractureproppant placement approaching a monolayer arrangement is .the mostdesirable. The tendency for the proppants to embed will however beincreased as the operator reduces the amount of proppant placed withinthe fracture. To maintain a high capacity fracture after subsequenthydrothermal treatments it therefore becomes necessary to condition, toa greater hardness, the fracture faces of the reservoir prior to thehydrothermal treatment.

This invention resides in la method of altering a reservoir martix t-odecrease fracture propp-ant embedment by injecting a heated mixture ofammonia hydrothermal treating fluid down the well bore and into afr-acture, and -continuing the injection until the formation is heatedto a minimum of 300 F. to a desired radial distance from the well boreand for a desired duration.

In the drawing:

The drawing represents a diagrammatic view of a partial vertical sectionof a well bore and the adjacent formation.

This drawing represents a well bore 2 extend-ing downward from thesurface of the earth, through a hydrocarbon-containing formation 4.Casing 6 is run into the 3,384,177 Patented May 21, 1968 well bore 2with cement 8 placed in the annulus formed between the face of the wellbore Z Iand the casing 6. The Well may be completed as an open-holecompletion with the casing 6 terminating at a point above thehydrocarbon-containing formation 4 or as a cased-through complet-ionwith the casing 6 terminating, as preferred in this invention and lasillustrated in the drawing, at a point bel-ow the hydrocarbon-containingformation 4. The upper end of the well will be equipped with n-ormalwell head equipment, to effect a closure of the well.

In this cased-through completion, opening 10, known as perforations,notches or windows, will be cut ythrough the casing 6 and theencompassing cement sheath 8. IFractures 12 will be produced through theformation 4 by various fracturing methods known to the oil industry.These fractures 4 will be propped in ian open position with proppants14.

To harden the matrix in the vicinity of the fracture faces a mixture ofheated .ammonia and a suitable hydrothermal treat-ing uid is injec-teddown the Well and into the hydrocarbon-containing formation 4 throughthe formation fractures 12. Hy'drothermal treating uid as used in thisinvention refers to any aqueous liquid utilized to heat the formation.It is preferred in this treatment to install tubing 16 through thecasing 6 and to seal the 4annulus created by the casing 6 and the tubing16 with a packer 18 positioned above the openings 10 to thehydrocarbon-containing form-ation 4.

Injection of the heated ammonia mixture is continued until the formationis heated a desired radial distance from the Well bore 2, preferably50-75 feet from the well bore. As the heated ammonia mixture is broughtin contact with the hydrocarbon-containing formation 4 various mineralscontained in the Iformation matrix 4 will undergo a conversion wherebycertain of these minerals crystalline form will be altered. This zone ofammonia treated matrix 24 exists as an area harder than the virginmatrix prior to ammonia treatment. The hardened matrix 24 will extend asuicient depth into the formation to support the overburden pressure`and prevent embedment of the proppants after hydrothermal treatment. Byso preventing embedment of the proppant the Width of the fracture 12will thereby be maintained.

In the formation treatment of this invention the injected heated ammoniaand hydrothermal treating fluid forms a 10.0- to 10.5 angstrom ammoniummica phase from the original reacting minerals of the formation matrix.In addition some analcite phases may be produced. These irreversiblemineral phases are non-expandable and will not cause matrix softeningwhen in contact with water or reservoir iiuids.

The ammonia utilized to form the ammonia mixture of this invention maybe injected in .the form of either gas or liquid. The hydrothermaltreating fluid which operates as the heating medium and transportingmaterial of the ammonia may be any fluid which an operator wouldnormally consider to use as a hydrothermal treating liquid. yExamples ofsuch fluids are steam, water, oilwater emulsions, and other aqueousfluids.

The variables encountered in utilizing this invention are thetemperature to which the matrix should be subjected, the ammoniaconcentration and the duration of treatment. lt has been found that theformation matrix to be hardened should be heated by the ammonia mixtureto a minimum temperature of 300 F. The amount of ammonia necessary toachieve the hardening reaction is found to be at least onemilliequivalent oframmonia per milliequivalent of montmorillonite whichwould be synthesized by hydrothermal treatment. This amount of ammoniain a treating fluid mixture must however be of sufficient concentrationto raise the pH of the mixture to at least 11.0. The minimum time toachieve an effective matrix hardening by this invention is thesubjection of the reservoir matrix to a minimum temperature of 300 F. ata minimum ammonia concentration for a minimum period of at least l2hours.

In a majority of reservoirs the preferred range of these variables wouldybe to subject the reservoir to a range of 0.02 to 0.05 pound of ammoniaper pound of hydrothermal treating fluid at a temperature between 500 to700 F. for a duration of 48 to 72 hours. These preferred ranges are notproposed to be limits for the variables of this treatment, but merelyare disclosed as preferred ranges for the operation of this invention.It has been found that any excess in the duration, concentration ortemperature, above the minimum limits prescribed, will enhance thetreatment.

The utilization of `water as the hydrothermal treating fluid and mixingthis water with a minimum amount of 0.013 pound of ammonia per pound ofwater will produce a fluid whose pH is in excess of 11.0. Any excess ofammonia above this amount will produce a treating mixture possessing ahigher pH. A treating mixture whose pH is lower than 11.0 will seriouslyreduce the effectiveness.

The method of treating fracture faces of this invention is applicablewhere a hydrothermal `treatment is to be performed on a fractured well.This invention more specifically is applicable in sandstone formations.

The various minerals constituting the formation matrix are theconstituents which give the matrix its hardness properties. Thishardness results from the cementation of the minerals within the matrix,the orientation of .the minerals crystalline structure and each mineralsown hardness property. Certain of these minerals will be caused to gointo solution as the heat and liquid of a hydrothermal treatment comesin contact with this matrix material. The minerals in solution will thenexist within the reservoir to form precipitants or aid in the synthesisof unwanted mineral structures.

When such mineral precipitants or unwanted mineral structures areproduced by hydrothermal atmospheres, the reservoir matrix is softenedto such an extent that the in place proppants embed in the fracturefaces when overburden loads are placed upon the fracture. This proppantembedment drastically reduces the permeability and porosity of thefracture. The flow capacity of the fracture is thereby reduced. In moreextreme softening conditions the fracture may completely close therebyseverely restricting the productivity of the well.

In formations containing silica, non-expandable clay forms and traces ofcarbonates, a hydrothermal treatment will dissolve a portion of theseminerals and convert a further portion into a new expandable form ofclay mineral called montmorillonite. As the montmorillonite t is formedthere will be a destruction of the clay or feldspar Imineral groups.Mineralogists term this reaction a hydrothermal synthesis ofmontmorillonite. This new synthesized montmorillonite, owing to itswater adsorption and lswelling properties, is expandable and possessessuch low structural hardness that proppants embed into the fracturefaces with resultant closure of the fracture.

Since the majority of hydrocarbon-containing formations contain mineralsthat are capable of such montmorillonite synthesis, it is advisable foran operator to prevent softening of the formation fracture faces. Theformations which will be softened by hydrothermal treatment may behardened and prevented from softening by the application of thisinvention. The ammonia treatment of this invention will also be usefulwhere it is desired to harden the matrix prior to placing proppants intoa fracture. In the event one or more of the minerals which causesoftening are not present in the formation, these missing minerals maybe added by injecting them, in a colloidal suspension or solution, into.the formation.

As an example of the hardening produced `by treating a reservoir matrixby this invention the following penetrometer hardness tests wereconducted on various sandstone cores which previously had been recoveredduring drilling operations.

These `tests were conducted by cutting a sample of the coreperpendicular to the bedding plane. Formation brine conditions weresimulated by saturating the core with a five percent sodium chloridesolution. A 0.072 inch ball point penetrometer was utilized to measurethe depth of penetration perpendicular to the bedding plane. Thepenetration depth of this ball joint penetrometer was directly recorded.

From the recorded penetration depth the diameter of the impression madeinto the core sample was calculated. A plot was then made on log logpaper of the diameter of impression divided by the diameter of the ballpoint penetrometer versus the load divided by the square of the diameterof the ball point penctrometer. The hardness of the sample therefore isthe value of the load divided by the square of the diameter of the ballpoint pentrometer corresponding to the point at which the diameter ofimpression divided by the diameter of the ball point pentrometer equalsone, this representing total embedment.

Stream treatment of the cores was accomplished by stream heating eachcore in a pressure bomb to 575 F. for a duration of 96 hours.

Ammonia treatment mixture of this invention was accomplished by treatingeach core in a pressure bomb. The cores were heated to 575 F. for 96hours by a steam mixture consisting of ammonia and water having a pH inexcess of 11.0.

In all tests it will be noted that the hardness of each treated core,after subsequent steam treatment, was greater than the hardness of theoriginal Virgin matrix.

Other tests have been conducted on cores using sodium and potassiumhydroxide solutions. It was noted in these tests that the rock washardened by both sodium treatments and potassium treatments. Thehardness achieved however was not retained as in the ammonia tests andboth the sodium and the potassium had upper concentration limits atwhich the rock was made softer than the virgin matrix.

I'IARDNESS TESTS ON SNDSTONI CORES The treatment of a fracturedhydrocarbon-containing formation to a minimum temperature of 300 F. withan ammonia mixture having a pH of at least 11.0 for a minimum durationof 12 hours will harden the formation matrix in the vicinity of thefracture faces to an extent that proppant embedment will be reduced. Bymaintaining the fracture width, after a subsequent hydrothermaltreatment, the operator may thereby maintain the flow capacity of thefracture without experiencing the detrimental effects of reducedpermeability or porosity which are generally associated withhydrothermal treatment.

Therefore we claim:

l. A method for altering the matrix of a hydrocarboncontaining formationto decrease further fracture proppant. embedment in a fracture having aproppant deposited therein, said fracture extending into the reservoirfrom a well penetrating the reservoir, said method cornprising injectinga heated mixture of ammonia and aqueous fluid containing at least 0.013pound of ammonia per pound of aqueous transporting fluid down the wellbore and into the formation and continuing injection until the formationmatrix adjacent the fracture is heated to a minimum of 300 F. to adesired radial distance from the well bore and for a period whereby theformation is hardened.

2. A method as set forth in claim 1 wherein the injection of the heatedammonia mixture is continued until the formation matrix adjacent thefracture is heated to a temperature of at least 300 F. to the radialextent of each fracture.

3. A method as set forth in claim 1 wherein the injection of the heatedammonia mixture is continued until the formation matrix adjacent thefracture is heated to a radial distance of 50 to 75 feet from the wellbore.

4. A method as set forth in claim 1 wherein the injection of the heatedammonia mixture is followed by a hydrothermal treatment, said injectionof ammonia mixture, and hydrothermal treating being continued to extendsubstantially the same distance from the well bore.

S. A method for hardening a reservoir matrix penetrated by a well boreextending from the surface of the earth downward through said matrix,comprising injecting a heated mixture of ammonia containing at least0.013 pound of ammonia per pound of aqueous transporting fluid down thewell bore and into the matrix, and continuing injection until theformatiion matrix desired to be hardened is heated to a minimumtemperature of 300 F. for a desired duration.

6. A method as set forth in claim 5 wherein the ammonia mixturepossesses a pH of at least 11.0.

7. A method for hardening, to minimize fracture proppant embedment, areservoir matrix penetrated by a well bore extending from the surface ofthe earth downward through said reservoir matrix comprising fracturingthe matrix, placing proppants within the fracture, and heating thematrix to a minimum temperature of 300 F. for a desired duration with aheated mixture of ammonia and aqueous fluid containing at least 0.013pound of ammonia per pound of aqueous transporting uid.

8. A method for altering, to decrease further fracture proppantembedment, a reservoir matrix penetrated by a well bore extending fromthe surface of the earth downward through said reservoir matrix, saidmatrix having a fracture extending through said reservoir matrix fromthe well and propping agents contained within the fracture maintainingthe fracture in an open position, comprising injecting a mixture ofammonia and an aqueous transporting uid containing at least 0.013 poundof ammonia per lpound of transporting fluid, injecting the mixture downthe well bore and into the formation at a temperature of at least 350F., and continuing injection until the formation matrix adjacent thefracture is heated to a temperature of at least 300 F. to a desiredradial distance from the well bore and for at least l2 hours.

9. A method for altering, to decrease further fracture proppantembedment, a reservoir matrix penetrated by a well bore extending fromthe surface of the earth downward through said reservoir matrix, saidmatrix having a fracture extending through the matrix from the well andpropping agents contained within the fracture, maintaining the fracturein an open position, comprising injecting a heated mixture of ammoniaand an aqueous transporting fluid containing at least 0.013 pound ofammonia per pound of transporting fluid, injecting the mixture down thewell bore and into the formation, continuing injection until theformation matrix adjacent the fracture is heated to a temperature in therange of from 500 to 700 F. to a desired radial distance, andmaintaining this temperature for a duration in excess of two days.

10. A method for altering, to decrease further fracture proppantembedment, a reservoir matrix penetrated by a well bore extending fromthe surface of the earth downward through said reservoir matrix, saidmatrix having a fracture extending through the matrix from the well andpropping agents contained within the fracture, maintaining the fracturein an open position, comprising mixing an aqueous treating fluid with atleast one milliequivalent of ammonia per milliequivalent ofmontmorillonite which would be synthesized by hydrothermal to produce anammonia mixture having a pH of at least 11.0, heating the ammoniamixture, injecting the heated ammonia mixture down the well bore andinto the formation, continuing injection until the formation matrixadjacent the fracture is heated to a temperature higher than 300 F. to adesired radial distance from the well bore, and maintaining thistemperature for a duration longer than two days.

11. A method as set forth in claim 6 wherein the ammonia mixturecontains 0.02 to 0.05 pound of ammonia per pound of transporting fluid.

12. A method for altering, to decrease further fracture proppantembedment, a reservoir matrix penetrated by a Well bol'e extending fromthe surface of the earth downward through said formation matrix, saidmatrix having a fracture extending through the matrix from the well andpropping agents contained within the fracture, maintaining the fracturein an open position, comprising mixing an aqueous treating fluid whichcontains more than 0.013 pound of ammonia per pound of water, heatingthe ammonia mixture, injecting the heated ammonia mixture down the wellbore and into the formation, continuing injection until the formationmatrix adjacent the fracture is heated to a temperature higher than 300F. to a radial distance at which any further hydrothermal treatments areexpected to be extended, and maintaining this temperature for a durationlonger than two days.

References Cited UNITED STATES PATENTS 3,302,718 2/1967 Prats 166--42 X3,323,594 6/1967 Huitt 166-42 3,343,600 9/1967 Phansalkar l66-423,349,351 l0/l967 Huitt 166-42 CHARLES E. OCONNELL, Primary Examiner.

NILE C. BYERS, IR., Examiner.

10. A METHOD FOR ALTERING, TO DECREASE FURTHER FRACTURE PROPPANTEMBEDMENT, A RESERVOIR MATRIX PENETRATED BY A WELL BORE EXTENDING FROMTHE SURFACE OF THE EARTH DOWNWARD THROUGH SAID RESERVOIR MATRIX, SAIDMATRIX HAVING A FRACTURE EXTENDING THROUGH THE MATRIX FROM THE WELL ANDPROPPING AGENTS CONTAINED WITHIN THE FRACTURE, MAINTAINING THE FRACTUREIN AN OPEN POSITION, COMPRISING MIXING AN AQUEOUS TREATING FLUID WITH ATLEAST ONE MILLIEQUIVALENT OF AMMONIA PER MILLIEQUIVALENT OFMONTMORILLONITE WHICH WOULD BE SYNTHESIZED BY HYDROTHERMAL TO PRODUCE ANAMMONIA MIXTURE HAVING A PH AT LEAST 11.0, HEATING THE AMMONIA MIXTURE,INJECTING THE HEATED AMMONIA MIXUTRE DOWN THE WELL BORE AND INTO THEFORMATION, CONTINUING INJECTION UNTIL THE FORMATION MATRIX ADJACENT THEFRACTURE IS HEATED TO A TEMPERATURE HIGHER THAN 300*F. TO A DESIREDRADIAL DISTANCE FROM THE WELL BORE AND MAINTAINING THIS TEMPERATURE FORA DURATION LONGER THAN TWO DAYS.